A magnetic disk device includes a magnetic disk for holding magnetic information, a slider on which a magnetic head for recording or reproducing the magnetic information in the magnetic disk is mounted, and an actuator rotatably provided to move the slider in the radial direction of the magnetic disk. The actuator has a suspension for holding the slider at the leading end side and applying a predetermined load on the slider and an arm for holding the base side of the suspension.
Among magnetic disk devices, there are magnetic disk devices each including an actuator having the mountable maximum number of suspensions (hereinafter, referred to as A) and magnetic disk devices each including an actuator having the smaller number of suspensions than that of those magnetic disk devices for reducing its storage capacity. This is to suppress the rise in manufacturing cost by using actuators having the same structure and adjusting the number of suspensions mounted thereon because the manufacturing cost of the actuators rises when the actuator structures vary depending on storage capacity of the magnetic disk device. In this case, in current magnetic disk devices, in place of suspensions, plates for providing mass balances of the actuators are attached to the arms with no suspension.
For example, in a magnetic disk device described in JP-A-2001-126428 (“Patent Document 1”), the frequency characteristic of an actuator is improved by attaching a weighting segment to an arm with no suspension to increase the lateral rigidity and mass of the weighting segment for equalizing the resonance frequency of the arm with no suspension and the resonance frequency of the arm with a suspension.
Since it is necessary for the suspension to flexibly support the slider, the rigidity of the suspension can not be made higher, and thus, the natural frequency in the disk radial direction can not be made so much higher. On the other hand, since the conventional plate has higher rigidity than that of the suspension, its natural frequency in the disk radial direction is significantly higher than that of the suspension. Accordingly, in the case of the actuator to which the conventional plate is attached in place of the suspension with the slider, the frequency characteristic of the actuator varies depending on the number of mounted suspensions and it is difficult to secure the equal positioning accuracy to that of the actuator with the maximum number of suspensions.